SUMMARY The clinical value of enzyme linked immunosorbent assays (ELISA) assays for IgM and IgG rheumatoid factors was assessed in a series of studies using rabbit IgG as antigen. The tests were reproducible with intra-assay coefficients of variation of 6% and could be simply and rapidly performed. Normal ranges were established using 106 sera from healthy controls. In a cross sectional study of 208 rheumatoid patients these assays were compared with the Rose-Waaler and laser nephelometric assessments of rheumatoid factor. In some patients there were discrepancies between rheumatoid factor positivity determined by one method or another. IgM ELISA and Rose-Waaler titres showed a significant correlation (r = +0 58; p < 0-001), but there was a low correlation between IgM and IgG ELISA (r = +0-27; p < 0-001). There was no evidence to show that the measurement of IgM or IgG rheumatoid factor gave significantly more clinical information than traditional tests such as the Rose-Waaler or latex agglutination tests.
A longitudinal study of changes in rheumatoid factor levels and their correlations with other clinical and laboratory assessment of disease activity followed 45 rheumatoid arthritis patients during 12 months continuous therapy with gold and penicillamine. Rheumatoid factors were measured by the titred latex assay and by ELISA assays for IgM and IgG rheumatoid factor. The IgM ELISA assay correlated strongly with the titred latex assay and weakly with the IgG rheumatoid factor assay at 0, 6 and 12 months. All showed significant falls with treatment, and ELISA assays were not more sensitive indicators than the latex assay. Correlations of initial rheumatoid factor levels by all 3 assays to 34 other clinical and laboratory assessments showed that only a small number of weak correlations existed; there were more at 6 and 12 months, but still not many. Changes in rheumatoid factor levels over 0-6 months showed no significant correlation to changes in other variables. There were no more correlations with ELISA assays than with titred latex rheumatoid factor. We conclude that ELISA rheumatoid factor assays have no clinically relevant advantage over the titred latex assay when following rheumatoid patients treated with gold or penicillamine.
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